It is generally recognized that steam radiators cannot efficiently transfer heat through the radiator walls if air is present in the heating medium. Numerous types of vent valves have been used with prior art steam radiators. Some of these valves rely on floating or thermally expansible elements to control venting of air. The prior art vent valves have been generally complex in construction and, hence, relatively expensive. Also, it is applicant's belief that existing vent valves have been designed specifically for use with continuously fired furnaces such as coal burning furnaces, wherein pressure within the radiators reaches two or more psig. Applicant has found that some prior art vent valves are not suitable for use with intermittently fired steam heating systems wherein lower pressures in the order of 1 psig occur; for example, air lock may develop and the radiator may fail to develop any appreciable heat, because the air lock inhibits the flow of heating medium from the furnace of the radiator.
A number of prior art vent valves are disclosed in U.S. Pat. Nos. 1,275,750; 1,778,612; 2,287,075 and 2,340,220.